Field of the Invention
This application relates to the field of steam generation.
Description of the Related Art
In the last 25 years, there has been a worldwide tendency towards increasing the efficiency and effectiveness of fossil-fired, in particular powdered-coal-fired, power plants in order to reduce emissions and thereby conserve fuel resources. To achieve this aim, it is necessary at the steam side to increase the steam parameters of pressure and temperature of the steam generator. The increase of said parameters of pressure and temperature leads to increased stresses in the tubes and in the tube walls of the steam generator. The walls which surround the combustion chamber of the steam generator and which are formed as evaporators or else as superheaters are often formed as diaphragm walls. Said walls are subjected to additional material stresses in that, during the erection of the diaphragm walls in the combustion chamber of the steam generator, further welding work must be performed here which could not be performed in the workshop beforehand. Owing to the size of the steam generators, it is additionally necessary here for diaphragms or webs to be welded to one another, or for diaphragms or fins/webs to be welded to tubes. Furthermore, openings which are formed, so-called windows, are welded by means of metal sheets.
Under the more stringent requirements, previously used materials such as the steel type T12 (ASTM 213) can no longer be used because these, owing to the technically relatively low stress loading capacity of the material, would necessitate very thick walls of the individual tubes of a tube wall, which would lead to a dramatic rise in the temperature required within the combustion chamber. To increase the admissible stress loadings of the material, further materials such as T22, T23 or T24 (ASTM 213) have therefore been developed. Said materials can in part be used without heat treatment after a welding process which is required during the erection of the steam generator. Problems have also been identified after a cleaning process, for example internal pickling of the tubes of a diaphragm wall. Depending on the application, the processing of said materials may result in the occurrence of stress corrosion cracking.
Stress-relief annealing heat treatment of tubular steel parts as structural components of body parts or chassis parts of a motor vehicle is known from DE 10 2005 033 360 A1.
Furthermore, it is known from WO 02/48411 A1 for cooling processes during the heat treatment of steel components to be controlled by virtue of media being sprayed or blown on.